With the increasing bandwidth demands from the advent of the Internet, service providers have looked for ways to increase data transmission performance over the copper wire local loop transmission lines that connect telephone central offices (COs) to customer premises (CPs). In conventional telephony networks, customer premises equipment (CPE) are coupled to CO switches over the above mentioned transmission lines, which are commonly known as “local loops,” “subscriber lines,” “subscriber loops,” “loops,” or the “last mile” of the telephone network. In the art, the term “line” and “loop” are used interchangeably, both terms referring to the copper wire pair used in a typical telephone transmission line conductor. Historically, the public switched telephone network (PSTN) evolved with subscriber loops coupled to a telephone network with circuit-switched capabilities that were designed to carry analog voice communications. “Central office” or “CO” means any site where a subscriber loop couples to a telephony switching unit, such as a public switched telephone network (PSTN), a private branch exchange (PBX) telephony system, or any other location functionally coupling subscriber loops to a telephony network. Digital service provision to the CP is a more recent development. With it, the telephone network has evolved from a system capable of only carrying analog voice communications into a system that can simultaneously carry voice and digital data.
Because of the prohibitive costs of replacing or supplementing existing subscriber loops, technologies have been implemented that utilize existing subscriber loops to provide easy and low cost migration to digital technologies. Subscriber loops capable of carrying digital signals are known as digital subscriber lines (DSLs). Various digital technologies provide customers with additional flexibility and enhanced services by utilizing frequency-division multiplexing (FDM) and/or echo-canceling (EC) and/or time-division multiplexing (TDM) techniques to fully exploit the transmission capability of a subscriber loop. These newer DSL technologies provide digital service to the customer premises without significantly interfering with the existing plain old telephone service (POTS) equipment and wiring by utilizing portions of the available frequency spectrum not used by a POTS signal. These portions of the frequency spectrum are often referred to as “logical channels.” Logical channels within a subscriber line that carry digital signals are known as “DSL channels,” while logical channels within a subscriber line which carry POTS analog signals are known as “POTS channels.”
DSL technologies, such as but not limited to integrated services digital network (ISDN), high-bit-rate digital subscriber line (HDSL), HDSL2 and symmetric digital subscriber line (SDSL), utilize echo-canceled pulse amplitude modulation to create a baseband data transmission spectrum and therefore do not coexist with a POTS signal which typically utilizes the 0–4 kilo-hertz (KHz) portion of the available frequency spectrum.
Other DSL technologies coexist with POTS by frequency-division multiplexing (FDM) a single data signal onto a logical channel above (at higher frequencies than) the 0 KHz to 4 KHz frequency range used by the analog POTS signals. Such multiplexing techniques and terminology are common to those skilled in the art, and are not described in detail herein. Examples of DSL technologies compatible with POTS include, but are not limited to, Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), Very High Speed DSL (VDSL), Multiple Virtual Lines (MVL™) and Tripleplay™. Communications systems employing DSL-over-POTS technology may frequency multiplex a plurality of data signals and a single POTS signal onto a single subscriber line. ADSL system employing time-division multiplexing would multiplex a plurality of data signals onto a single logical channel with each different data signal allocated to a predefined portion of time in a predefined, repeating time period. Note that an advantage of TDM is that the transmitter does not actively transmit at all times.
FIG. 1 is a simplified illustrative block diagram of a portion of an existing telephony system 20 which includes a telephone company central office (CO) 22 coupled to communication system network 24 and coupled to a customer premises (CP) 26 via a single subscriber loop 28. Subscriber loop 28 may be any suitable connection for communicating electrical signals, but is typically a copper wire pair, as is well known in the art, that was originally designed to carry a 0–4 KHz analog voice channel (POTS signal). When a copper wire pair is used for data signal transmission, the wire pair is often referred to as a digital subscriber loop (DSL).
Located within the CO 22 is the digital transmitter unit 30, signal front end system 32 and power supply 34. Digital transmitter unit 30 includes at least a transmitter signal generating circuitry 36 and a transmitter 38. Signal front end system 32 detects incoming communication signals from network 24, via connection 46, which are to be transmitted to CP 26. Signal front end system 32 performs the necessary signal processing of the communication signal received from network 24, and passes the communication signal to transmitter signal generation circuitry 36, via connection 48. Transmitter signal generation circuitry 36 further process the communication signal; a common example known to those skilled in the art is a DSP (Digital Signal Processor). Such processing may include modulation of the communication signal for transmission to customer premises 26. Transmitter signal generation circuitry 36 passes the processed communication signal to transmitter 38, via connection 50. Transmitter 38 provides the necessary communication signal amplification so that a communication signal having the proper signal strength can be transmitted, via connection 52, onto subscriber loop 28.
Power supply 34 provides the necessary power to transmitter 38, via connection 54. Also, power supply 34 provides power to components residing in the transmitter signal generation circuitry via connection 54. Power supply 34 also provides power to other components residing in the CO 22, however, such connections providing power to these other components are not shown for convenience of illustration.
Many other components typically reside in CO 22 which are not illustrated in FIG. 1 for convenience. For example, no digital receiver circuitry, POTS signal circuitry, couplers between the POTS and the digital systems are shown in FIG. 1. Such components are not described in detail herein as these components are well known in the art. Furthermore, not all of the components residing in the signal front end system 32, the transmitter signal generating circuitry 36 or transmitter 38 are described herein in detail or illustrated in FIGS. 1–8 other than to the extent necessary.
Located within the CP 26 may be a plurality of digital equipment devices which transmit and receive data signals over subscriber loop 28. For convenience of illustration, a personal computer (PC) 40 is shown residing in CP 26 and coupled to subscriber loop 28. Illustrative examples of other digital equipment devices include, but are not limited to, facsimile (FAX) machines, set top boxes, internet appliances, computers or the like. PC 40 includes a modem (not shown), or the like, coupled to subscriber loop 28. PC 40 may communicate with a plurality of other digital equipment devices (not shown) via an Ethernet (not shown), other local access network (LAN), or the like (not shown). PC 40 includes user interface devices, such as keyboard 42 and/or viewing screen 44, to interface with a user (not shown).
A modem (not shown), typically residing in PC 40, decodes a data signal received from the digital transmitter unit 30 over the subscriber loop 28. The modem also transmits communication signals onto the subscriber loop 28 which have been generated by PC 40 or other similarly functioning digital device residing in the CP 26. Typically data is communicated using a communication signal that has been modulated. Modulation schemes used to communicate between CO 22 and CP 26 may include, but are not limited to, carrierless amplitude/phase modulation (CAP), quadrature amplitude modulation (QAM), Discrete Multi Tone (DMT) or pulse amplitude modulation (PAM), and are commonly known in the art and are not described in detail herein.
Prior art digital communication systems, like the signal front end system 32 and the digital transmitter unit 30 illustrated in FIG. 1, are often added into an existing CO 22 so that the digital communication system can utilize existing POTS facilities, such as power supplies, building structures, grounding and protection facilities, etc. Also, it may be desirable to expand already existing digital communication facilities residing in the CO 22. However, electrical code requirements, regulations and/or rules pertaining to the heat generated by digital communication system components may limit the size of the digital communication system addition or expansion. Such code requirements specify the maximum heat generation allowed per unit size of floor space and/or per unit size of cabinet volume. In other situations, limited physical space may be available within the CO 22 for digital communication system additions or expansions. Consequently, a more compact construction of the digital communication system components may be desirable. Also, available power supplies and the load carrying capacity of existing facilities, which provide the power to the digital communication system additions or expansions, may be limited. Therefore, it is desirable to reduce power consumption in at least some of the components of a digital communication system. Reducing power consumption would facilitate a more compact construction of an electrical code compliant digital communication system addition or expansion.